1. Field of the Invention
This invention relates to a system for locating salient objects or features contained within a pattern, particulary, though not exclusively, in a static image or a video sequence. The invention also includes an image compression system.
2. Related Art
The human visual eye-brain perceptual system is very good at identifying the most important features of a scene with which it is presented, or the identification of objects that are different in some respect from the background or surrounding population, without the prior training required of most automated systems. However, there are some applications where automation is desirable, for example where the work is very repetitive and the volume of data is very large. A particular example is the inspection of medical smear samples in order to identify cancerous cells. In such situations, where a large number of samples have to be inspected and anomalies are rare, a human observer may become inattentive, and overlook the very feature being searched for.
A system that automatically identifies distinctive objects in an image would also be desirable for many other purposes, for example the identification of the location of the principal subject in a visual scene, the design and location of information signs, and as a substitute for lengthy and expensive human factors trials of visual display equipment.
Existing systems for gauging visual attention extract previously specified features (e.g. colour, intensity, orientation) from images, and then train classifiers (e.g. neural networks) to identify areas of high attention. These trainable models rely heavily on the selection of the features to be searched for in the image, and have no way of handling new visual material that has little similarity with that used to design and test the system. Paradoxically, a feature may simply be too anomalous to be identified as such by a trained system. Such systems also require considerable computational resource in order to process the pre-selected features and moreover this burden increases without limit as the scope of the procedure is extended and more features are added.
The majority of known image compression systems have the disadvantage that they can only compress images with a constant compression rate and thus constant compression quality. Known variable rate compression systems cannot automatically vary the compression rate according to the regions of interest in the image. In most cases, it would be sufficient to compress only regions of interest with high quality while compressing the rest of the image (such as the background) with low quality only. As compression quality and image file size are dependent upon each other, this would reduce the total amount of space required for the compressed image file. One of the techniques used by professional Web designers is to simply blur the background of images before compressing them with JPEG. This forces the background to be made up of continuous tones thus reducing the amount of high spatial frequencies in the image. Images that are pre-processed that way can have their storage requirements reduced by up to 30% depending on the amount of blurring compared to non-blurred images. Blurring images by hand is very labour intensive and depending on the image it might not save enough space to be worth doing.
Joint Picture Experts Group is working on a new image compression standard, JPEG 2000, which also allows specifying regions of interest in images to compress them with higher quality than the rest of the image. However, automatic identification of regions of interest is still a problem.